Tire treads for pneumatic tires typically have running surfaces of a singular, unitary, rubber composition and consistent physical properties across the face of the tread intended to be ground contacting.
Often the tire tread may be of a cap/base construction composed of an outer tread cap layer presenting the running surface of the tire and an underlying tread base layer as a transition between the tread cap layer and the tire carcass. The tread cap layer itself may be of a lug and groove configuration with the outer surface of the lugs, including lugs in a from of ribs, themselves presenting the running surface of the tire tread. Such overall tire tread cap/base construction is well known to those having skill in such art.
For example, an all-season tire tread cap layer may be of an individual rubber composition designed to present a tread running surface for a balance of a combination of wet traction, cold weather winter traction (for snow and/or ice), dry handling, and resistance to tread wear properties.
However, optimizing one or more individual tread properties such as, for example, wet traction, cold weather winter traction, dry handling and resistance to tread wear properties typically requires a compromise of one or more of the other tread properties.
Tires have been heretofore proposed which have circumferentially zoned treads for various purposes including a desire to present a plurality of individual running surfaces with various characteristics from one tire tread. For example, see U.S. Pat. Nos. 4,319,620, 4,385,653, 6,474,382, 6,540,858 and 6,959,744; U.S. Patent Application Nos. 2002/0033212, 2004/0112490 and 2005/0167019. European patent publication Nos. 0341187, 0662396, 0839675 and 1308319; WO99/01299; and Japanese Patent Publication Nos. 2001/047815 and 85/60135309.
Tire treads have heretofore been mentioned as having running surfaces composed of three longitudinal portions namely, two black colored lateral portions and a non-black colored central portion located between the two black portions, wherein the lateral black colored portions have wear resistant properties virtually identical to the central colored portion (for example: EP0993381 A3, FR2765525 and WO99/01299 patent publications).
In U.S. Pat. No. 5,225,011, a tire is presented having a tread composed of a center rubber composition and side rubbers (its FIG. 1) configured as being positioned directly onto a tire carcass belt without a tread base transition layer. The center rubber is illustrated as being composed of natural rubber or a natural rubber/styrene-butadiene rubber blend. The center rubber contains a carbon black of large iodine absorption number of at least 100 mg/g, silica and silane coupling agent and the side rubbers are required to be of a different rubber composition.
In European patent publication number EP864446 A1 a tire is presented, for example, as having a tread (its FIG. 2) with a central portion (B) occupying at least 37 percent of the tread surface and side portions (A) positioned directly onto a tire carcass belt without a tread base transition layer. The side portions may primarily contain, for example, carbon black reinforcement and the central portion may primarily contain, for example, silica reinforcement, wherein the silica content of the central portion (B) is at least 20 percent higher than in the side portions (A).
For the zoned tread cap layer of this invention the tread cap zones are capable of being load-bearing in a sense that the primary and outboard tread cap segments, or zones, extend radially inward from the outer surface of the tread cap layer to the underlying tread base layer rubber composition so that the load on the tire may be communicated by the tread cap layer zones to the transitional tread base layer instead of directly to remainder of the tire carcass itself.
For this invention, it is desired to present an outer tread cap layer with a running surface comprised of two individual circumferential load-bearing zones for the running surface of the tread which exhibit one or more graduated physical properties, and which extend from the outer running surface of the tread cap layer radially inward to said tread base layer.
The tread cap layer is asymmetrical in the sense of the aforesaid primary tread zone and said lateral control element zone of significantly different widths for the span of the tread running surface in a manner that the primary tread cap zone is not centered over the centerline, or equatorial plane, of the tire.
In practice, the significantly narrower lateral outboard control element circumferential tread zone, when the tire is mounted on a rim to form a tire/rim, or wheel, assembly, for an associated vehicle is intended to be positioned axially outward, or outboard, insofar as the associated vehicle is concerned and therefore is referred to herein as an outboard element of the tread cap.
The tread cap is primarily a silica-rich rubber tread cap load bearing layer which contains a minor axially positioned outboard control element, or portion, comprised of a carbon-black rich, silica-containing, rubber composition. For this invention it is intended to maximize the portion of the running surface of the tread cap which is a silica-rich rubber composition and use the relatively minimal axially outboard control element to add a control aspect to the silica-rich primary tread layer (e.g. dry handling and resistance to tread wear by the said lateral outboard tread zone intended to be positioned outboard from the associated vehicle body and therefore referred to herein as the outboard tread cap zone in the sense of being intended to be positioned outboard in relation to its positioning with the associated vehicle) to the silica-rich primary tread layer.
Accordingly, for this invention, the tread cap is required to be primarily composed of a primary cap layer which contains an inclusion of a relatively minimal axially positioned lateral outboard control element which extends over a maximum of only about 30, preferably a maximum of only about 26 percent, of the span of the free running surface of the tread cap in a form of a carbon black-rich, silica-containing, rubber load bearing tread zone, or element, in order to substantially maintain the tread cap layer in a form of a primary silica-rich rubber component which extends over at least 70, preferably at least about 74 percent, of the span of the free running surface of the tread cap.
Regarding elastomer compositions and reinforcing filler for respective primary and outboard control tread cap zones; in practice, the rubber composition of the primary, silica-rich tread cap component, or zone, contains a high reinforcing filler content composed of a combination of precipitated silica and rubber reinforcing carbon reinforcement, with a high content of precipitated silica, in combination with a relatively high content of low Tg (glass transition temperature) cis 1,4-polybutadiene rubber (e.g. about −100° C. to about −106° C.) to promote wet traction (promote tread traction for wet road conditions) and handling under winter conditions (e.g. ice and snow). This is in combination with use of a tin coupled amine functionalized styrene/butadiene elastomer and preferably a capped organoalkoxymercaptosilane coupler for the precipitated silica to aid in coupling the precipitated silica to the diene-based elastomers to promote a reduced viscosity (Mooney ML+4 viscosity property at 100° C.) during the processing (mixing) of the rubber composition.
The rubber composition of the axially outboard control element, component or zone, of the tread cap layer, in one aspect of the invention, contains a reduction of the low Tg polybutadiene rubber content as well as a reduction of the reinforcing filler content (combination of precipitated and rubber reinforcing carbon black), but with a significant increase of the carbon black/silica ratio, to cause the rubber composition to be carbon black-rich (in a sense of the reinforcing filler being primarily composed of the carbon black) to promote dry traction (promote tread traction and tire handling for dry road conditions), as well as resistance to wear, and in combination with the tin coupled, amino functionalized styrene/butadiene elastomer and preferably a capped organomercaptoalkoxysilane coupler for the precipitated silica to aid in coupling the precipitated silica to the diene-based elastomers and in providing a reduced viscosity (Mooney ML+4 viscosity property at 100° C.) during the processing (mixing) of the rubber composition.
Regarding physical properties of the respective primary and outboard control tread cap zones; in practice, it is desired for the cold storage modulus (G′) at −25° C. of the rubber composition of the primary tread cap zone to be significantly lower than that of the rubber composition of the outboard control tread cap element, or zone, (for example, by at least 6 MPa and preferably at least about 8 MPa or more) in order that the rubber composition of the primary tread cap zone is relatively softer (not as stiff in the dynamic storage modulus G′ sense) than the outboard tread cap control element to promote winter performance.
Also, in practice, it is desired for the warmer dynamic storage modulus (G′) at 60° C. of the rubber compositions of the primary tread cap component and outboard tread cap control element to be similar (for example, within about 2 MPa of each other) in order that they have a similar stiffness (in a sense of the dynamic storage modulus G′ property) to promote handling for non-winter, dry road conditions.
Further, in practice, it is desired that the rubber composition of the outboard tread cap control element, or zone, is more hysteretic than the rubber composition of the primary tread cap zone, in a sense of its tan delta, 100° C. (3 percent strain and 10 Hertz) in order to promote dry handling grip for the tire (e.g. dry traction when turning the vehicle). Accordingly, in practice, it is preferable that such tan delta property of the rubber composition of the outboard control tread cap element, or zone is at least 8 percent, and preferably at least 10 percent, greater than such tan delta property of the rubber composition of the primary tread cap zone (e.g. is therefore more hysteretic in nature).
Accordingly, the rubber compositions of the strategically positioned primary rubber tread cap and lateral outboard control element of the tread cap layer present a cooperative combination of graduated physical properties across the running surface of the tire in a sense of dynamic storage modulus (G′) at +60° C. and at −25° C., as well as the aforesaid tan delta property, for the individual primary zone and control element tread cap rubber compositions.
It is therefore considered herein that one aspect of the invention is for a tire tread cap layer comprised of said primary (silica-rich, relatively high reinforcement-containing and relatively high content of low Tg polybutadiene rubber) rubber tread cap zone and said asymmetrical lateral outboard (carbon black-rich, lower reinforcement-containing and reduced content of low Tg polybutadiene rubber) control tread cap element, or zone, that the aforesaid cold dynamic storage modulus G′ (at −25° C.), hot tan delta (at 100° C.) and warm dynamic storage modulus G′ (at 60° C.) physical properties relating to the said tread cap layer zones, or segments, are assembled, or combined, in a cooperative manner, particularly by the strategic positioning of outboard control element segment on the outboard position of the primary tread cap zone, to provide the overall tread running surface with suitable gradations of physical properties relating to the aforesaid cold weather winter handling as well as non-winter driving conditions.
The term “axial width” of a free running surface of a tread of a tire, or tread cap layer, means the “axial span of the free running surface” of the tread cap layer, or zone, unless otherwise indicated, as the axial span of the outer surface of such tread cap layer (inflated tire at 75 percent of standard load) which is intended to be ground-contacting under normal straight, (without the vehicle turning), driving conditions, where the axial span includes (spans across) any grooves in the outer surface of the tread cap layer which are not normally intended to be ground contacting. The tire inflation pressures versus standard loads for pneumatic tires may ordinarily be found in the Tire and Rim Association, Inc Year Book to determine a standard load for a tire at a chosen inflation pressure (usually the highest inflation pressure listed). For passenger tires, an inflation pressure of 35 psi (240 kPa), is used and for light truck tires (LT) an inflation pressure of 50 psi (343 kPa) is used.
The term “axial width of a total running surface” of a tread of a tire, or tread cap layer, means the “axial span of the total running surface” of the tread cap layer, unless otherwise indicated, as the axial span of the total outer surface of such tread cap layer (usually the front vehicular tire) which is intended to include intermittently ground-contacting when the vehicle is turning, which normally includes said “axial span of the free running surface” and outer portions of both sides of the tread surface which is designed to be intermittently ground-contacting and the included space across the opening of any tread grooves (spans across the grooves) contained in such tread cap layer running surface.
In general, the axial span of the total running surface may be, for example, from about up to about 15, for example from about 5 to about 15, percent greater than the axial span of the free running surface, depending somewhat of the tire's tread cap design as well as the size of the tire and tread itself.
When a tread cap zone is referenced herein as axially spanning a free running surface or total running surface of the tread cap, unless otherwise indicated, such axial span extends axially, or laterally, across such running surface in a direction perpendicular to the equatorial plane (EP) of the tire and therefore does not include the curvature of the tread, or tread cap, itself.
In the description of this invention, the terms “rubber” and “elastomer” where herein, are used interchangeably, unless otherwise provided. The terms “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art. The terms “cure” and “vulcanize” may be used interchangeably unless otherwise provided. In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer.
The term “Tg”, where used herein, refers to the glass transition temperature of an elastomer, which may normally be determined by a differential scanning (DSC) calorimeter with a temperature rise of 10° C. per minute, (ASTM 3418), a method well known to those having skill in such art.
The dynamic storage modulus (G′) viscoelastic property, as well a tan delta property (100° C.), of a rubber composition may be obtained using a ARES™-LS2 rheometer from the TA Instruments company of New Castle, Del. (USA) and equipped with a liquid nitrogen cooling device and forced convection oven to allow testing of rubber samples over a broad temperature range below and above ambient temperature. A cylindrical cured rubber sample is used which is approximately 8 millimeters in diameter and approximately 2 millimeters in height glued between two brass cylinders of approximately 8 millimeters in diameter. Such glue may, for example, be a cyanoacrylate based glue. Orchestrator™ software is used to control the ARES™-LS2 rheometer. Using said software, the temperature increase rate of 5° C. is set. A temperature sweep at 3 percent torsional strain and 10 Hertz frequency of from, for example, about −30° C. to about +60° C. is used in which the dynamic storage moduli (G′) values may be determined over said temperature range. From said determined dynamic storage moduli (G′), observations are made at −25° C. and +60° C. Use of dynamic storage modulus (G′) viscoelastic property to characterize various aspects of cured rubber compositions is well known to those having skill in such art.